A double azimuth 4-head type video tape recorder is well known in the field.
That is, a revolving drum faces a running recording medium, i.e., a video tape, at an inclined angle. This drum includes a pair of standard speed recording/regenerating heads which are oppositely disposed and separated by 180 degrees. In addition, the drum includes, on the outer circumferential surface thereof, a pair of low speed recording/regenerating heads which are separated by 180 degrees, and also separated by a small distance (about 2 minutes horizontal scanning distance) from the above mentioned standard heads. In total, 4 heads consisting of two pairs of standard speed and low speed recording/regenerating heads forming two sets respectively are installed separated by 180 degrees on the drum.
Of the pair of the standard speed recording/regenerating heads, one of them has +6 degrees azimuth (VHS method) or +7 degrees azimuth (B method) (the azimuth indicating the angle between the head running direction and the lengthwise direction of the magnetic cap of the head). Against this angle, the other head forms an azimuth of -6 degrees or -7 degrees. Thus there is seen a difference between the two azimuths adjacent to each recorded video track for forming a picture by means of the traces of the relative motions between the video tape and the heads.
That is, a video track is recorded in an azimuth of +6 degrees or +7 degrees by one of the heads, and the adjacent video track is recorded in an azimuth of -6 degrees or -7 degrees by the other head. Thus, during a regenerating operation of a single track, although a head partially faces adjacent tracks cross talk arising therefrom can be effectively inhibited. This is a known technique (NHK home video technique, Japanese Broadcasting Association Item 87).
The pair of the low speed heads are disposed on the circumferential surface of the drum and mutually separated by 180 degrees. One of them is disposed adjacent to one of the standard speed heads having an azimuth of +6 degrees or +7 degrees has an azimuth of -6 degrees or -7 degrees. The other low speed head is disposed adjacent to the other standard speed head having an azimuth of -6 degrees or -7 degrees has an azimuth of +6 degrees or +7 degrees.
Therefore, a variable speed regeneration can be carried out using the standard speed head thereby assuring low speed regeneration or a high speed regeneration of a video track of a video tape recorded in a standard speed (this will be called standard speed recorded medium). This is achieved by using the pair of the standard speed heads. In a variable speed regeneration operation, the recorded video track and the head scanning track do not correspond to each other, and therefore, the adjacent video track is included during the regeneration scanning.
Therefore, the low speed head of a narrow width (19f) (VHS method), facing a video track having a wide width (58f) (VHS method) and recorded by a standard speed head, can partially regenerate a wide width video track which is recorded with the corresponding azimuth inclination angle. This regeneratable portion almost corresponds to the portion which is impossible to regenerate by means of the standard speed head. Therefore, when a video tape which is recorded in the standard speed is to be regenerated, the picture data which corresponds to the unregeneratable portion during a variable speed regeneration is replaced with the regeneration picture data obtained from a low speed head which is simultaneously operating.
Thus the required compensation is achieved with this well known technique. (Television technology, electronic technology publication, volume of April, 1989, Items 54-58).
Assuming such a double azimuth 4-head type, and assuming the possibility of recording/regeneration by the two pairs of the heads, in the case where the recorded data is subjected to a variable speed regeneration using the pair of the standard speed heads, the stripe shaped or strip shaped optical noise can be replaced with a substitution picture data. This conventional method is illustrated in FIG. 6.
Referring to FIG. 6, a pair of standard speed regenerating heads SP1,SP2 are connected through a pair of separate pre-amplifiers 1,2 to an enlarged region analogue switching circuit 3 which is commonly used as a shifting means for the standard speed regenerating head. In the same way, a pair of low speed regenerating heads EP1,EP2 are connected through a pair of pre-amplifiers 4,5 to an enlarged region analogue switching circuit 6 which is well known and commonly used as a switching means for the low speed regenerating head, with this switching circuit 6 being operated so as to complement the switching circuit 3.
In addition to the illustrated components, the analogue switching circuits 3,6 receive regenerating head switching pulses SWP and compensating pulses SWP in a separately expressed order, so that the two switching circuits 3,6 should be able to perform compensatory switching operations. In relation with this, an example for the regenerating head switching pulses SWP is taken. That is, a state of "0" is maintained for the angular interval of 180 degrees where a first standard speed regenerating head SP1 and a second low speed regenerating head EP2 are facing to the video tape which performs relative motions.
As against this, the analogue switching circuit 3 switches the first standard speed regenerating head SP1 to the standard speed regenerating signal VSP1 through the pre-amplifier 1 in order to output a standard speed regenerating signal VSP. Meanwhile, the compensation regenerating head switching pulse SWP which is inverted by an inverter (not shown) is maintained in a state of "1", while, in response to this, the analogue switching circuit 6 switches a low speed regenerating signal VEP2 for the second low speed regenerating head EP2 so that a low speed regenerating signal VEP to be outputted.
Successively to the above operation, an inverse relation is established such that the regenerating head switching pulse SWP is in a state of "1", and the compensation regenerating head switching pulse SWP is in a state of "0", for the angular interval of 180 degrees of the drum in which the second standard speed regenerating head SP2 and the first low speed regenerating head EP1 are facing to the video tape which is performing relative motions. Consequently, the two analogue switching circuit 3,6 switches the standard speed regenerating signal VSP2 for the second standard speed regenerating head SP2 and the low speed regenerating signal VEP for the first low speed regenerating head EP1 in a separately expressed order before outputting them.
Thus the output terminals of the two analogue switching circuits 3,6 which switch the regenerating signals VSP,VEP are respectively connected to the input terminal of a next standard speed regenerating envelope detecting circuit 7 and to the input terminal of a low speed regenerating envelope detecting circuit 8 in a separately expressed order. Meanwhile, the output terminals of the two envelope detecting circuits 7,8 are connected to first and second input terminals of a comparator 9.
The output terminal of the comparator 9 is connected to operate an analogue switching circuit 10, while the switching circuit 10 is connected to a compensation regenerating signal output terminal 11 by selecting one of the output terminals of the analogue switching circuit 3 and the analogue switching circuit 6 in response to the regeneration signal switching pulse SSP of the comparator 9. Thus the switching circuit 10 forms a means for switching the regeneration signal for the standard speed recorded medium.
Now will be considered the case where picture data of a standard recording using the standard recording heads SP1,SP2 is subjected to a variable speed regeneration at triple speed using the low speed regenerating heads EP1,EP2. The operation of the conventional apparatus will be described referring to FIGS. 7A and 7B. FIG. 7A illustrates an overlapping of the scanning traces of the video head with the video tracks for the recording time during a triple speed regeneration, i.e., the scanning traces of the video head obtained by observing the relative motions of the video head from the non-magnetic face of the video tape. FIG. 7B illustrates the wave pattern of the conventional apparatus in time series.
Where video tracks a,b,c recorded by standard recording heads are subjected to a variable speed regeneration of a triple speed as shown in FIG. 7A, video head scanning traces d,e are formed by the standard speed regenerating head during the variable speed regeneration. Here, a video head scanning trace e is is produced by the second standard speed regenerating head SP2, while another video head scanning trace e' shown in dotted lines and having a narrow width is produced by the first regenerating head EP1.
The mutual positional relationship between the video head scanning traces e,e' is governed by the standard speed of the known double azimuth 4-head type paired heads, and by the widths and position heights of the low speed regenerating video heads (i.e., the heads SP2,EP1). During the recording, the axial inclinations of the scanning traces e,e' are steeper by about 3 times that of the axial inclinations of the standard speed video tracks a,b,c. The reason is that, in the variable speed regeneration of a triple speed, the regeneration is carried out at a speed 3 times the standard recording speed. The above principle is validly applied also to video head scanning traces d,d' which are succeeded in time series.
However, during the time when the second standard speed regenerating head SP2 and the first low speed regenerating head EP1 are moved in a fixed relation in accordance with the scanning traces e,e' of the two video heads which precede in time series, the regeneration head switching pulse SWP lies in a state of "1", and the compensating pulse for it lies in a state of "0" as already described [FIG. 7B (A)f]. In response to this, the analogue switching circuits 3,6 switch the regenerating signals VSP2,VSP1 of the second standard speed regenerating head SP2 and the first low speed regenerating head EP1, before outputting them.
The detail of the video head scanning trace e produced by the second standard speed regenerating head SP2 will be described for the case where the regeneration head switching pulse SWP is restored to a period of state "1" [FIG. 7B(A)f] and a variable speed regeneration is carried out in this period. The initial period of the relative motion between the tape and head forms a wedge shaped region XSP2 having a narrowing tail. As against this wedge shaped region, a gradually reducing regenerating signal VSP2 [FIG. 7B (B)h] is produced by the second regenerating head SP2.
A lozenge region YEP1 following the wedge shaped region XSP2 has an azimuth which does not correspond with that of the second standard speed regenerating head SP2, and, therefore, the regenerating signal VSP2 disappears [FIG. 7B(B)i]. Instead, the azimuth of the first low speed regenerating head EP1 which is moving simultaneously comes to be corresponded with the lozenge region YEP1, and, therefore, it gradually increases correspondingly with the lozenge region to stay at a certain value, while the lozenge shaped regenerating signal VEP1 [FIG. 7B(C)j] is regenerated by the first low speed regenerating head EP1.
Further, as against a wedge shaped region ZSP2 which has a thick tail, and which succeeds the lozenge region YEP1, the second standard speed regenerating head SP2 produces a regenerating signal VSP2 [FIG. 7B(B)k] which gradually increases correspondingly with the wedge shape having a thick tail. During the interval where the regeneration head switching pulse SWP stays in a state of "0" all in the same way, the first standard speed regenerating head SP1 and the second low speed regenerating head EP2 which are switched perform relative motions in accordance with the video head scanning traces d,d' succeeding in time series.
First, as against a wedge shaped region XSP1 having a thin tail, a gradually decreasing regenerating signal VSP1 [FIG. 7B(B)i] is regenerated by the first standard speed regenerating head SP1, and then, as against the lozenge shaped region YEP2, a lozenge shaped regenerating signal VEP2 [FIG. 7B(C)m] is regenerated by the second low speed regenerating head EP2. Then, as against the wedge shaped region ZSP1 having a thick tail, a gradually increasing regenerating signal [FIG. 7B(B)n] is regenerated by the first standard speed regenerating head SP1.
Thus, during the period when the regeneration head switching pulse SWP is in a state of "1", the regenerating signals VSP2 which are regenerated by the second standard speed regenerating head SP2 are supplied through the above described analogue switching circuit 3 to the standard speed regeneration envelope detecting circuit 7 in the form of standard speed regenerating signals VSP. Then the circuit 7 outputs standard speed regeneration envelope signals ESP which expresses the envelope of the regenerating signals VSP detected as above.
The regenerating signals VEP1 which are regenerated by the first low speed regenerating head EP1 are supplied through the above described analogue switching circuit 6 to the low speed regeneration envelope detecting circuit 8 in the form of low speed regenerating signals VEP. The circuit 8 outputs low speed regeneration envelope signals EEP expressing the envelopes of the regenerating signals VEP detected as above.
Then the succeeding comparator 9 receives the standard speed regeneration envelope signals ESP and the low speed regeneration signals EEP through the first and second input terminals, respectively, and then, compares the magnitudes of the two sets of the signals. Under this condition, during the period in which the regeneration head switching pulses SWP is in a state of "1", the gradually decreasing regeneration video signals VSP of the second standard speed regenerating head SP2 and the gradually increasing regeneration signals VEP1 of the first low speed regenerating head EP1 cross each other.
Then at the time point where the two values become equal each other [FIG. 7B(C)o], the relation of the magnitudes of the two regenerating envelope signals ESP,EEP are inverted, with the result that the regenerating envelope signals EEP become larger than the regenerating envelope signals ESP. Consequently, the regeneration signal switching pulse SSP which is for the standard speed recorded medium and which is outputted by the comparator 9 is shifted to a state of "1", and, in response to this, the analogue switching circuit 10 is switched over
In other words, the selected ones are inserted into the second standard speed regenerating video signals VSP2 [FIG. 7B(D)p], and then, the mixed signals are supplied to the compensated regenerating signal output terminal 11 in the form of compensated regenerating signals VSSP for the standard speed recorded medium.
During the period in which the succeeding regeneration head switching pulse SWP is in a state of "0", the second low speed regenerating signals VEP2 having greater envelopes are inserted into the first standard speed regenerating signals VSP1 [FIG. 7B(D)q] based on the same operation as described above, and then, the mixed signals are supplied to the output terminal 11 in the form of compensated regenerating signals VSSP for the standard speed recorded medium.
However, in the case where a picture data which is completely recorded by means of a low speed recording head is taken as the object of the concern, the compensating operations for continuous regenerations bring various undesirable results. Such undesirable phenomena will be described referring to FIG. 8A (which is equivalent to FIG. 7A), FIG. 8B (which is equivalent to FIG. 7B) and FIG. 9, which precisely enlarges the recess of the wave pattern.
Now it is assumed that the video tracks a,b,c, . . . i,j,k which have narrow widths and are recorded by low speed recording heads are continuously regenerated at a triple speed. Then, during the variable speed regeneration, video head scanning traces l,m having wide widths are formed by standard speed regenerating heads. Further, these wide video head scanning traces l,m, include video head scanning traces 1', m' having narrow widths and formed by the low speed regenerating heads which are operated simultaneously with the standard speed regenerating heads during the variable speed regeneration.
First, in response to the regeneration head switching pulse SWP [FIG. 8B(A)n] which is in a state of "1", the first low speed regenerating head EP1 which is performing a variable speed regeneration of a triple speed makes relative motions along the video head scanning trace m' which precedes in time series. As against the wedge shaped region QEP1 having a narrow end and appearing in the initial stage of the relative motions, the first low speed regenerating head EP1 regenerates regenerating signals VEP1 [FIG. 8B(B)o] having a gradually decreasing trend in correspondence with the wedge shaped region having a narrow end.
Following the wedge shaped region QEP1, the azimuth of a lozenge shaped region RSP2 does not correspond with that of the first low speed regenerating head EP1, and therefore, the regenerating signals VEP1 disappear [FIG. 8B(B)p]. In the meantime, there occurs an overlapping with the leading end portion of the wedge shaped region QEP1 having a narrow end, and therefore, there is regenerated a regenerating signal VEP1 [FIG. 8B(B)q] having a gradually increasing trend in correspondence with a wedge shaped region having a wide end.
In the meantime, the second regenerating head SP2 which performs relative motions along the video head scanning trace m regenerates strip shaped regions [(RSP2)+(R'SP2)] which have leading end portions divided from the narrow ended wedge shaped region TSP2 by the video head scanning trace m as an inclined boundary.
Further, in correspondence with a wide ended wedge shaped region USP2, the second regenerating head SP2 further regenerates another signal [FIG. 8B(C)r] having a substantially flat form, while, in correspondence with the inclined cut portion of the region R'SP2, the head SP2 regenerates a gradually decreasing regenerating signal VSP2 [FIG. 8B(C)s]. In the above cases, the azimuths of the regions QEP1,SEP1 do not correspond with that of the second standard speed regenerating head SP2, and therefore, the regenerating signal VSP2 is not implicated here.
Then, if the head switching pulse SWP is shifted to a state of "0.revreaction., then, in response to this, the first standard speed regenerating head SP1 and the second low speed regenerating head EP2 start the operations of a variable speed regeneration of a triple speed. Then they perform relative motions along the wide width video head scanning trace 1 which succeeds in time series. Thus, as against the regions PSP1, WEP2,XSP1, YEP2, ZSP1, the first standard speed regenerating head SP1 and the second low speed regenerating head EP2 respectively regenerate two regenerating signals VSP1,VSP2 [FIG. 8B(B)t, FIG. 8B(C)u] which increase and decrease in the same form as when the head switching pulse SWP is in a state of "1".
Thus, in the case where a picture data which is recorded in a low speed by the low speed recording heads EP1,EP2 is to be subjected to a variable speed regeneration, it is very difficult to form compensated regenerating signals simply by comparing the magnitudes of the envelopes of the regenerating signals VEP,VSP, as is apparent by the increasing/decreasing trends of the regenerating signals VEP,VSP as shown in FIGS. 8B and 8C. This is the principal difference from the case where a picture data which is recorded at a standard speed by means of the standard speed recording heads SP1,SP2 is subjected to a variable speed regeneration.
Further, in the case where a picture data which has been recorded at a low speed by means of the low speed recording heads is subjected to a variable speed regeneration, the low speed regenerating heads EP1,EP2 have to be justly used to obtain regenerating signals VEP, if a proper picture data is to be obtained. Therefore, its use usually should be confined to the case where the signals are damped by the regenerating signals VEP to such an extent that partial degradation of the picture data or cross talk occur.
Therefore, it is more desirable in terms of the picture quality to use the regenerating signals VSP which are regenerated by the standard speed regenerating heads SP1,SP2. However, the current regenerating signals VEP are very low in their signal level compared with the regenerating signals VSP which are regenerated by the standard speed regenerating heads SP1,SP2. Therefore, there exist differences in the envelopes of the regenerating signals VEP1, VEP2 regenerated by the first and second low speed regenerating heads EP1, EP2, while there exist also step differences between the levels of the wave centers of the two regenerating signals VEP1, VEP2.
Consequently, due to the current regenerating signals VEP regenerated by the first and second low speed regenerating heads EP1, EP2 and due to their envelopes, there are many difficulties in properly deciding the time of selecting the regenerating signals VSP which are regenerated by the first and second standard regenerating heads SP1, SP2.
FIG. 9 illustrates the wave patterns showing mainly the time relation between the regeneration signal switching pulses SEP for the low speed recorded medium, which are used in deciding the time of selecting the regenerating signals VEP1, VEP2 of the first and second low speed regenerating heads EP1, EP2 and the regenerating signals VSP1, VSP2 of the first and second standard speed regenerating heads SP1, SP2. The above described difficulties will be described in further detail referring to FIG. 9.
In this drawing, during the period when the regeneration head switching pulse SWP is in a state of "1", and when the first low speed regenerating head EP1 is performing a variable speed regeneration at triple speed [FIG. 9(A)a], the regenerating head EP1 obtains a regenerating signal VEP1 through the pre-amplifier 4, and this regenerating signal VEP1 has an amplitude-modulated wave pattern [FIG. 9(B)b] which is equivalent to the main body of the envelope of one side as shown in FIG. 8B schematically and extractively.
Meanwhile, during the time when the regenerating head switching speed regenerating head EP2 is performing continuously a variable speed regeneration at triple speed [FIG. 9(A)c], this time, the switched second low speed regenerating head EP2 obtains another amplitude-modulated wave pattern [FIG. 9(B)d] through the pre-amplifier 5, with this wave pattern succeeding the wave pattern of the first low speed regenerating head EP1.
Now a comparison will be made on the separate amplitude-modulated wave patterns [FIG. 9(B)b, FIG. 9(B)d] of the first and second low speed regenerating heads EP1, EP2. Generally speaking, the wave pattern (amplitude) [FIG. 9(B)V] of the first low speed regenerating head EP1 and the wave pattern [FIG. 9(B)W] of the second low speed regenerating head EP2 do not correspond each other, and particularly, the center position of the wave pattern [FIG. 9(B)X] of the former and the center position of the wave pattern [FIG. 9(B)Y] of the latter do not correspond.
FIG. 9C illustrates a state which is formed by extracting the envelopes of the regenerating signals VEP of the low speed regenerating heads EP1, EP2, after taking into account the above described non-correspondences. According to the wave pattern of this drawing, during the first half period as a state of "1" and during the second half period as a state of "0", the shape of the envelope, particularly the rising and falling slopes [FIGS. 9(C)e and 9(C)f] and the position of the envelope, particularly its minimum value [FIGS. 9(C)g and 9(C)h] exist in different forms. Because of these different existences, difficulties are encountered in the following period.
That is, this refers to a period in which the case is different from that of the regenerating signals VSP of the first and second standard regenerating heads SP1,SP2, and in which the case of the regenerating signals VEP of the first and second low speed regenerating heads shows a significant relative damping in the envelopes of the regenerating signals VEP of the low speed regenerating heads EP1,EP2.
In other words, this refers to the period in which the relative damping is significantly seen in the envelopes EEP of the regenerating signals VEP as shown in FIG. 9D, i.e., in the regeneration signal switching pulse (corresponding to SSP of FIG. 6) required for deciding the period for compensating the cross talks and degradations of the regenerating picture data by inserting the regenerating signals VSP of the simultaneously operating standard speed regenerating heads SP1,SP2.
That is, during this period, it is very difficult to form the regeneration signal switching pulse SEP for a low speed recorded medium by inverting to a state of "0" [FIG. 9(D)i]. Therefore, in the case where a variable speed regeneration is carried out using the low speed regenerating heads EP1,EP2 and using the low speed recorded medium, the stripe shaped or optical noises which appear on the regenerated picture can not be eliminated.